System and method for detecting diode failures

ABSTRACT

A system for detecting faults in a rectifier includes an AC current generator and a rectifier. A controller is configured to determine an AC input current supplied to rectifier, generate a simulated rectified DC input current based upon the AC input current, and determine a DC output current from the rectifier. The controller is further configured to compare the simulated rectified DC input current to the DC output current and generate an alert command if a difference between the simulated rectified DC input current and the DC output current exceeds a predetermined difference threshold.

TECHNICAL FIELD

This disclosure relates generally to AC power rectification systems and,more particularly, to a system and method for detecting the failure ofone or more diodes within a rectifier.

BACKGROUND

Machines that utilize electric power often include a power generationsystem having a prime mover and a generator for generating theelectrical power. The generator may be configured as an alternator thatgenerates AC electrical power. In many instances, it is desirable toconvert the AC electrical power into DC power through the use of arectifier system.

A fault within an AC power generation system may result in theunbalanced generation of AC power that may damage components of thepower generation system. In one example, certain types of faults mayresult in overheating of the windings of one or more phases within analternator. Once a fault has been detected, a technician is often calledupon to locate and fix the fault and return the machine to operation.

Systems have been developed to assist the technician by determining thetype of fault that has occurred within the power generation system. Forexample, a sequence transformer system may be utilized to transform theoutput of the alternator into a positive sequence of phasors, a negativesequence of phasors, and a zero sequence of phasors. Properties of thesequences may be analyzed using symmetrical component analysis todetermine the type of fault that has occurred. For example, certainproperties may indicate a ground fault and other properties may indicatea fault between phases. In addition, other properties may indicate thenumber of phases between which a fault has occurred.

Knowing the type of fault may reduce the time required for a technicianto locate and thus fix the fault. However, the sequence transformersystem may not identify the location of the fault within the powergeneration system. As a result, a technician may be required to spend aconsiderable amount of time attempting to identify the specific locationof the fault.

Relay protection systems may be used to determine whether a fault hasoccurred in an electrical component and to shut down the current to thecomponent or other systems to protect the component and the systems.Differential relays often operate by comparing the input current to theoutput current and triggering the relay if the difference exceeds athreshold for a predetermined time. The difficulty in using a relay tomonitor the operation of a rectifier is increased due to the differentforms of input and output current (i.e., AC current and DC current,respectively).

U.S. Pat. No. 7,994,798 discloses a power generation system thatincludes an alternator and a rectifier for converting AC power to DCpower. Current sensors may be used to measure the DC current that isprovided to the electric motors of a traction system. A system isprovided to test the current sensors by comparing a measured currentwith a stored profile. The test results may be used to assist indetermining the location of a fault.

The foregoing background discussion is intended solely to aid thereader. It is not intended to limit the innovations described herein,nor to limit or expand the prior art discussed. Thus, the foregoingdiscussion should not be taken to indicate that any particular elementof a prior system is unsuitable for use with the innovations describedherein, nor is it intended to indicate that any element is essential inimplementing the innovations described herein. The implementations andapplication of the innovations described herein are defined by theappended claims.

SUMMARY

In one aspect, a system for detecting faults in a rectifier includes anAC current generator for generating an AC output current and a rectifierincluding at least one diode operatively connected to receive the ACoutput current to define an AC input current to the rectifier andtransform the AC input current into a DC output current. A first currentsensor is configured to generate first current signals indicative of theAC input current and an output current sensor is configured forgenerating output current signals indicative of the DC output current. Acontroller is configured to store a predetermined difference threshold,receive the first current signals from the first current sensor,determine the AC input current based upon the first current signals, andgenerate a simulated rectified DC input current based upon the AC inputcurrent. The controller is further configured to receive the outputcurrent signals from the output current sensor, determine the DC outputcurrent based upon the output current signals, compare the simulatedrectified DC input current to the DC output current, and generate analert command if a difference between the simulated rectified DC inputcurrent and the DC output current exceeds the predetermined differencethreshold.

In another aspect, a controller-implemented method of detecting faultsin a rectifier having at least one diode includes storing apredetermined difference threshold, generating an AC output current,providing the AC output current to the rectifier to define an AC inputcurrent, and transforming the AC input current into a DC output currentthrough the rectifier. The method further includes receiving firstcurrent signals from a first current sensor indicative of the AC inputcurrent, determining the AC input current to the rectifier based uponthe first current signals, and generating a simulated rectified DC inputcurrent based upon the AC input current. The method also includesreceiving output current signals from an output current sensorindicative of the DC output current from the rectifier, determining theDC output current based upon the output current signals, comparing thesimulated rectified DC input current to the DC output current, andgenerating an alert command if a difference between the simulatedrectified DC input current and the DC output current exceeds thepredetermined difference threshold.

In still another aspect, a machine includes an AC current generator forgenerating an AC output current, a prime mover operatively connected tothe AC current generator, and a rectifier including at least one diodeoperatively connected to receive the AC output current to define an ACinput current to the rectifier and transform the AC input current into aDC output current. A first current sensor is configured to generatefirst current signals indicative of the AC input current and an outputcurrent sensor is configured for generating output current signalsindicative of the DC output current. A controller is configured to storea predetermined difference threshold, receive the first current signalsfrom the first current sensor, determine the AC input current based uponthe first current signals, and generate a simulated rectified DC inputcurrent based upon the AC input current. The controller is furtherconfigured to receive the output current signals from the output currentsensor, determine the DC output current based upon the output currentsignals, compare the simulated rectified DC input current to the DCoutput current, and generate an alert command if a difference betweenthe simulated rectified DC input current and the DC output currentexceeds the predetermined difference threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a machine in which the principlesdisclosed herein may be used;

FIG. 2 is a block diagram of a portion of the machine of FIG. 1including a power generation system;

FIG. 3 is a flowchart of a simulated rectifier system;

FIG. 4 is a flowchart of a process for determining the type and locationof a fault within a three-phase power generation system;

FIG. 5 is an exemplary graph of a simulation of three-phase AC inputcurrents generated by an alternator;

FIG. 6 is an exemplary graph of a simulated rectified DC input currentbased upon the AC input currents of FIG. 5;

FIG. 7 is an exemplary graph of a simulation of a DC output current froma rectifier based upon the input current of FIG. 5;

FIG. 8 is an exemplary graph of a difference between the simulatedrectified DC input current of FIG. 6 and the simulation of the DC outputcurrent of FIG. 7;

FIG. 9 is an exemplary graph of a second simulation of three-phase ACinput currents generated by an alternator;

FIG. 10 is an exemplary graph of a simulated rectified DC input currentbased upon the AC input currents of FIG. 9;

FIG. 11 is an exemplary graph of a simulation of a DC output currentfrom a rectifier based upon the input current of FIG. 9; and

FIG. 12 is an exemplary graph of a difference between the simulatedrectified DC input current of FIG. 10 and the simulation of the DCoutput current of FIG. 11;

DETAILED DESCRIPTION

FIG. 1 is a diagrammatic illustration of a machine 10 that may be usedin accordance with an embodiment of the disclosure. The machine 10 mayinclude a chassis 11 that supports a prime mover such as an engine 12and a cab 13 in which an operator may be positioned. The engine 12 maybe operatively connected to and drives one or more ground engaging drivemechanisms such as wheels 14. More specifically, engine 12 may beoperatively connected to an AC current generator or alternator 20 (FIG.2) to energize two inverter systems (not shown). The inverter systemsmay supply current to two electric motors (not shown) to drive thewheels 14.

A control system 15 as shown generally by an arrow in FIG. 1 indicatingassociation with the machine 10 may be provided to control the operationof the machine. The control system 15 may include a plurality of sensorsas shown generally by arrow 16 and an electronic control module such ascontroller 17. The plurality of sensors 16 may operate by providing dataor signals indicative, directly or indirectly, of the performance orconditions of various aspects of the machine 10. The controller 17 mayreceive operator input command signals and control the operation of thevarious systems of the machine 10.

The controller 17 may be an electronic controller that operates in alogical fashion to perform operations, execute control algorithms, storeand retrieve data and other desired operations. The controller 17 mayinclude or access memory, secondary storage devices, processors, and anyother components for running an application. The memory and secondarystorage devices may be in the form of read-only memory (ROM) or randomaccess memory (RAM) or integrated circuitry that is accessible by thecontroller. Various other circuits may be associated with the controllersuch as power supply circuitry, signal conditioning circuitry, drivercircuitry, and other types of circuitry.

The controller 17 may be a single controller or may include more thanone controller disposed to control various functions and/or features ofthe machine 10. The term “controller” is meant to be used in itsbroadest sense to include one or more controllers and/or microprocessorsthat may be associated with the machine 10 and that may cooperate incontrolling various functions and operations of the machine. Thefunctionality of the controller 17 may be implemented in hardware and/orsoftware without regard to the functionality. The controller 17 may relyon one or more data maps relating to the operating conditions of themachine 10 that may be stored in the memory of controller. Each of thesemaps may include a collection of data in the form of tables, graphs,and/or equations. The controller 17 may use the data maps to maximizethe performance and efficiency of the machine 10.

Referring to FIG. 2, a block diagram of a portion of machine 10 isdepicted including the engine 12 operatively connected to a three-phasepower generation system 18 that includes alternator 20 that generatesalternating voltage. An AC bus 21 is operatively connected to alternator20 and includes three power lines 22 (such as cables) depicted as afirst phase power line A for conducting the first phase of theelectricity from the alternator 20, a second phase power line B forconducting the second phase of the electricity from the alternator, anda third phase power line C for conducting the third phase of theelectricity from the alternator. As depicted, alternator 20 generatesthree-phase electrical energy including three-phase AC output currentwith one phase or phase A of AC output current being transmitted alongthe first phase power line A, a second phase or phase B beingtransmitted along the second phase power line B, and a third phase orphase C being transmitted along the third phase power line C.

The power lines 22 conduct AC output current from alternator 20 torectifier 25. Rectifier 25 may include a plurality of diodes 26 or otherelectronic devices and is configured to convert AC current to DCcurrent. As depicted, rectifier 25 includes six diodes 26 with twodiodes connected in series to each phase of the AC bus 21 as athree-phase full-wave bridge rectifier circuit. Rectifier 25 isconfigured to convert the AC input current (which is AC output currentfrom the alternator 20) to a single DC output current that is providedalong a single DC output line 27 (such as a cable) to DC bus 28. DC bus28 may be operatively connected to a DC load 29 such as the inverters(not shown) used to convert DC power to AC power to drive the wheels 14of the machine 10. A DC return line 33 extends from the DC load 29 tothe rectifier 25.

Although depicted as having three phases, the alternator 20 andrectifier 25 may have any desired number of phases. In addition, in someinstances, rectifier 25 may have other configurations.

Each phase of the power lines 22 may include a current sensor 30associated therewith for measuring the AC input current through therespective power lines. Accordingly, a first current sensor, a secondcurrent sensor, and a third current sensor are depicted in FIG. 2. Inaddition, the DC output line 27 may also include an output currentsensor 31 for measuring the output current exiting from the rectifier25. Each of the current sensors 30 and the output current sensor 31 maybe part of control system 15 and are operatively connected to controller17 to provide current data or current signals to the controller. Thecurrent sensors 30 and the output current sensor 31 may be any type ofsensor that will measure the current through the respective lines. Thecontroller 17 may determine the AC input current in power lines 22 basedupon current signals from current sensors 30 and the DC output currentin DC output line 27 based upon current signals from the output currentsensor 31. In one example, the current sensors 30 and the output currentsensor 31 may be Hall effect sensors. Other types of sensors may alsoused. For example, when measuring the current in the AC power lines 22,the current sensors 30 may be current transformers.

Under most circumstances, the power generation system 18 may operate asdesired and the current generated by alternator 20 and conducted bypower lines 22 to rectifier 25 may operate in a balanced manner (i.e.,the current along each power line is equal and the phases displaced orseparated by equal angles). However, in some instances, the powergeneration system 18 may operate in an unbalanced manner which may beindicative of a problem within the system. In order to assist inlocating problems within the power generation system 18, controller 17may include a sequence transformer system 32 as is known in the art thatoperates to transform the characterization of the output of thealternator 20 into a positive sequence of phasors, a negative sequenceof phasors, and a zero sequence of phasors. More specifically and usingthe current generated by the alternator 20 as an example, the sequencetransformer system 32 resolves the generated current into a positivesequence set of equal currents (I₁) that is displaced and rotatesaccording to the output current along each power line 22, a negativesequence set of equal currents (I₂) that is displaced and rotates in adirection opposite the positive sequence, and a zero sequence set ofequal currents (I₀) in which has a zero phase displacement from theothers.

According to symmetrical component analysis, the presence of onlypositive sequence currents indicates a balanced power system within thephases. In such case, the negative sequence and the zero sequencecurrents are equal to zero. If the negative sequence currents are notequal to zero, a fault such as a short circuit exists on or between oneof the phases. If the zero sequence currents are not equal to zero, aground fault exists between one or more of the phases and a groundreference.

While the sequence transformer system 32 will identify the type of faultwithin an electrical system, a significant amount of work by atechnician may be required to find the exact location of the fault as itmay exist anywhere within the system. Accordingly, the controller 17 mayinclude a diode failure detection system 35 that operates to determinewhether a fault that is identified is located within or outside therectifier 25.

The diode failure detection system 35 may include a simulated rectifiersystem 36 and a simulated relay system 37. The simulated rectifiersystem 36 operate by determining the actual AC input current to therectifier and generating a simulated rectified DC input current basedupon such AC input current. More specifically, referring to theflowchart in FIG. 3, the controller 17 may receive at stage 40 currentsensor data or current signals from each current sensor 30 associatedwith the power lines 22. At stage 41, the controller 17 may determinethe current in each power line 22 based upon the current sensor datafrom each current sensor 30.

The controller 17 may determine at stage 42 the absolute value of thecurrent within each power line 22 based upon the data from each currentsensor 30. The controller 17 may add the absolute values of the currentstogether at stage 43. If desired, a scaling factor may be applied atstage 44 to the sum of the absolute values of the currents to create asimulated rectified DC input current that more accurately reflects theactual amount of current that would be rectified if a second physicalrectifier were operatively connected to the output from the currentsensors 30 associated with the power lines 22. In the example depictedin FIG. 2 with a three-phase system, a scaling factor of 0.5 may be usedwith the sum of the absolute values being multiplied by the scalingfactor. Other scaling factors may also be used.

The simulated relay system 37 may operate in a manner similar to aconventional physical relay system (not shown) in which the actual inputcurrent flowing into and the actual output current flowing out of adevice are compared to determine whether the component is operatingproperly. More specifically, the simulated relay system 37 may comparethe simulated rectified DC input current determined by the simulatedrectifier system 36 to the actual DC output current determined by theoutput current sensor 31 on the DC output line 27 exiting the rectifier25. If the difference between the simulated rectifier DC input currentand the DC output current is greater than a predetermined differencethreshold for longer than a threshold period of time, the controller 17may simulate tripping or actuating a relay. The controller 17 maygenerate an alert command to an operator of the machine 10, store anerror code within the controller, shut down one or more components ofthe machine or the entire machine, and/or take any other desiredactions.

The difference threshold may be set to any desired value based upon thedesired sensitivity of the simulated relay system 37. Whiletheoretically any difference between the input current and the outputcurrent may indicate a fault, a non-zero difference threshold istypically preferred to avoid tripping or actuating the relay based uponerrors or tolerances that may occur during operation that are notrelated to a fault within the relay. In one example in which the ratedcurrent of the alternator is 950 A, the difference threshold may be setto 5 A which is approximately 0.5% of the rated current. In otherapplications, other thresholds may be used. For example, a if a greatersensitivity is desired, the difference threshold may be as low as 0.1 to0.3% of the rated current.

The threshold period of time may be set to ensure that a fault hasoccurred before tripping or actuating the relay. In one example, thethreshold period of time may be set at twenty milliseconds. Otherthreshold periods of time may be used depending on the desiredsensitivity of the simulated relay system 37.

By combining the sequence transformer system 32 with the diode failuredetection system 35, the controller 17 may determine the type of faultwithin the power generation system 18 and also determine whether thefault is within the rectifier 25 or outside of the rectifier such aswithin the alternator 20 or along the power lines 22.

Referring to FIG. 4, a flowchart is depicted of a process fordetermining the type of fault within the three-phase power generationsystem 18 and whether the fault is within the rectifier or outside ofthe rectifier. At stage 50, the controller 17 may analyze the AC inputcurrent delivered to the rectifier 25 over power lines 22 and determinethe equivalent or simulated DC input current to the rectifier asdescribed above with respect to FIG. 3.

At stage 51, the controller 17 may receive data from the output currentsensor 31 associated with the DC output line 27 and determine the actualDC output current exiting from the rectifier 25 along the DC outputline. At stage 52, the controller 17 may perform a symmetrical componentanalysis on the current passing through each power line 22 to determinethe positive sequence set of equal currents I₁, the negative sequenceset of equal currents I₂, and the zero sequence set of equal currentsI₀.

The simulated relay system 37 of controller 17 may determine at stage 53the difference between the simulated DC input current and the actual DCoutput current. At decision stage 54, the simulated relay system 37 maydetermine whether the difference between the simulated DC input currentand the actual DC output current is greater than a difference thresholdfor a duration that exceeds a threshold period of time.

If the difference between the simulated DC input current and the actualDC output current exceeds the difference and time threshold at decisionstage 54, a fault exists in the rectifier circuit within rectifier 25.At decision stage 55, the controller 17 may determine whether the zerosequence set of equal currents I₀ is equal to zero. If the zero sequenceset of equal currents I₀ is zero, the controller 17 may determine atdecision stage 56 whether the negative sequence set of equal currents I₂is equal to zero. If the negative sequence set of equal currents I₂ isequal to zero, the fault within the rectifier 25 may be either betweenthe diodes of three phases or between the diodes of the three phases andalso a ground reference as depicted at stage 57. If the negativesequence set of equal currents I₂ is not equal to zero, the fault withinthe rectifier 25 is between the diodes of two of the phases as depictedat stage 58.

If the zero sequence set of equal currents I₀ is not equal to zero atdecision stage 55, the controller 17 may determine at decision stage 59whether the positive sequence set of equal currents I₁, the negativesequence set of equal currents I₂, and the zero sequence set of equalcurrents I₀ are all equal. If the positive sequence set of equalcurrents I₁, the negative sequence set of equal currents I₂, and thezero sequence set of equal currents I₀ are all equal, the fault withinthe rectifier 25 is between one of the diodes and the ground referenceas depicted at stage 60. If the positive sequence set of equal currentsI₁, the negative sequence set of equal currents I₂, and the zerosequence set of equal currents I₀ are not all equal at decision stage59, the controller 17 may determine at decision stage 61 whether the sumof the positive sequence set of equal currents I₁, the negative sequenceset of equal currents I₂, and the zero sequence set of equal currents I₀equals zero. If the sum of the positive sequence set of equal currentsI₁, the negative sequence set of equal currents I₂, and the zerosequence set of equals currents I₀ equals zero, the fault within therectifier 25 is between two of the diodes and also the ground referenceas depicted at stage 62. If the sum of the positive sequence set ofequal currents I₁, the negative sequence set of equal currents I₂, andthe zero sequence set of equal currents I₀ does not equal zero, thefault within the rectifier 25 is that one of the diodes is shorted asdepicted at stage 63.

Referring back to decision stage 54, if the simulated DC input currentand the actual DC output current are within the difference and timethresholds, the power generation system 18 may be working properly or afault may exist outside of the rectifier 25. At decision stage 64, thecontroller 17 may determine whether the zero sequence set of equalcurrents I₀ is equal to zero. If the zero sequence set of equal currentsI₀ is zero, the controller 17 may determine at decision stage 65 whetherthe negative sequence set of equal currents I₂ is equal to zero. If thenegative sequence set of equal currents I₂ is equal to zero, thecontroller 17 may determine at decision stage 66 whether the positivesequence set of equal currents I₁ is greater than a threshold currentlevel or value. The threshold current level may be set at any desiredvalue that defines a threshold as to whether the power generation system18 is operating properly. As an example, if the alternator 20 is ratedat 950 A, the threshold current level may be set at 1900 A. In suchcase, it may be expected that the power generation system 18 may beoperating properly if the current at times exceeds 950 A but is lessthan 1900 A.

If the positive sequence set of equal currents I₁ is greater than thethreshold current level at decision stage 66, a fault exists between thethree phases on either the alternator 20 or the power lines 22 asdepicted at stage 67. If the positive sequence set of equal currents I₁is less than the threshold current level, no faults exist in thealternator 20 or the power lines 22 and the power generation system 18is operating properly as depicted at 68. If the negative sequence set ofequal currents I₂ is not equal to zero at decision stage 65, a faultexists within either the alternator 20 or the power lines 22 and thefault is between two of the phases as depicted at stage 69.

If the zero sequence set of equal currents I₀ is not equal to zero atdecision stage 64, the controller 17 may determine at decision stage 70whether the positive sequence set of equal currents I₁, the negativesequence set of equal currents I₂, and the zero sequence set of equalcurrents I₀ are all equal. If the positive sequence set of equalcurrents I₁, the negative sequence set of equal currents I₂, and thezero sequence set of equal currents I₀ are all equal, a ground faultexists between one phase of either the alternator 20 or the power lines22 and the ground reference as depicted at stage 71. If the positivesequence set of equal currents I₁, the negative sequence set of equalcurrents I₂, and the zero sequence set of equal currents I₀ are not allequal at decision stage 70, the controller 17 may determine at decisionstage 72 whether the sum of the positive sequence set of equal currentsI₁, the negative sequence set of equal currents I₂, and the zerosequence set of equal currents I₀ equals zero. If the sum if thepositive sequence set of equal currents I₁, the negative sequence set ofequal currents I₂, and the zero sequence set of equal currents I₀ equalszero, a ground fault exists between two phases of either the alternator20 or the power lines 22 and the ground reference as depicted at stage73. If the sum of the positive sequence set of equal currents I₁, thenegative sequence set of equal currents I₂, and the zero sequence set ofequal currents I₀ does not equal zero, there is an unknown fault asdepicted at stage 74.

FIGS. 5-8 depict a simulation of a first example of stages 50-54 inwhich the power generation system 18 has no faults and is operating in abalanced manner. FIG. 5 depicts a simulation of the current in eachpower line 22 with the first phase power line A depicted at 100, thesecond phase power line B depicted at 101, and the third phase powerline C depicted at 102. An example of the simulated DC input currentdetermined at stages 40-44 of FIG. 3 is depicted at 103 in FIG. 6. Anexample of the DC output current from rectifier 25 is depicted at 104 inFIG. 7. FIG. 8 depicts that the difference 105 between the simulated DCinput current 103 and the DC output current 104 is essentially zero andthus there are no faults in the rectifier 25.

Upon determining that a fault does not exist in the rectifier 25, thecontroller 17 may utilize symmetrical component analysis to determinewhether a fault exists elsewhere within the power generation system 18(such as within the alternator 20 or the power lines 22) and, if so, thetype of fault by following stages 64-74. It should be noted that thesymmetrical component analysis will not identify whether a fault existsin the alternator 20 or the power lines 22 but only that a fault existswithin the power generation system 18. Further, based upon the analysisof the difference between the simulated DC input current and the DCinput current in this simulation, it is known that any such fault is notwithin the rectifier 25.

FIGS. 9-12 depict a simulation of a second example of stages 50-54 inwhich the power generation system 18 has a fault between one of thephases within the rectifier 25 and a ground reference and thus thesystem is operating in an unbalanced manner. FIG. 9 depicts a simulationof the current in each power line 22 with the first phase power line Adepicted at 106, the second phase power line B depicted at 107, and thethird phase power line C depicted at 108. An example of the simulated DCinput current determined at stages 40-44 of FIG. 3 is depicted at 109 inFIG. 10. An example of the DC output current is depicted at 110 in FIG.11. FIG. 12 depicts the difference 111 between the simulated DC inputcurrent 109 and the DC output current 110 which exceeds the differenceand time thresholds and therefore a fault exists within the rectifier25.

Upon determining that a fault exists in the rectifier 25, the controller17 may utilize symmetrical component analysis to determine the type offault within the rectifier by following stages 55-63.

Although depicted in FIG. 4 and described herein in the context ofcurrents and sums of currents being equal or equal to zero, thecontroller 17 may not require exact equivalence so as to compensate fortolerances associated with the power generation system 18. Morespecifically, the operation of the sequence transformer system 32 andthe diode failure detection system 35 may not require exact equivalence.More specifically, the systems may utilize various non-zero thresholdsrather than requiring exact equivalence.

Referring to FIG. 4, the simulated relay system 37 may be configured torequire a difference between the simulated DC input current and theactual DC output current to exceed at decision stage 54 a predeterminednon-zero threshold or difference threshold. As depicted in FIGS. 8 and12, the simulated relay system 37 is configured to trip or activate thesimulated relay if the difference between the simulated DC input currentand the DC output current exceeds 5 A. In the simulated examples ofFIGS. 5-8 and 9-12, the alternator 20 is rated at 950 A and thereforethe difference threshold is set at approximately 0.5% of the ratedcurrent. If a more sensitive simulated relay were desired, the simulatedrelay system 37 may be configured to trip or activate the simulatedrelay with a smaller difference threshold such as 1 to 3 A which isapproximately 0.1-0.3% of the rated current.

In one example, at decision stages 55 and 64, the threshold for the zerosequence set of equal currents I₀ may be set at approximately 10% of therated current. In another example, at decision stages 56 and 65, thethreshold for the negative sequence set of equal currents I₂ may be setat approximately 5% of the rated current. Either larger and smallerthresholds may be utilized for the zero sequence set of equal currentsI₀ and/or the negative sequence set of equal currents I₂ in somesituations.

In still another example, at decision stages 59 and 70, the condition ofthe decision stages may be met if the positive sequence set of equalcurrents I₁, the negative sequence set of equal currents I₂, and thezero sequence set of equal currents I₀ are all within or do not vary bymore than approximately 5-10%. While larger and smaller thresholds arepossible, in some instances a range of approximately 20% may be toolarge. In a further example, at decision stages 61 and 72, the conditionof the decision stages may be met if the sum of the positive sequenceset of equal currents I₁, the negative sequence set of equal currentsI₂, and the zero sequence set of equal currents I₀ is withinapproximately 5% of the rated current.

INDUSTRIAL APPLICABILITY

The industrial applicability of the system described herein will bereadily appreciated from the foregoing discussion. The foregoingdiscussion is applicable to machines that include a rectifier 25 as partof a power generation system 18.

Symmetrical component analysis may be used to determine the type offault within a power generation system 18. However, a technician mayneed to spend a significant amount of time locating the fault even onceaware of the type of fault. The diode failure detection system 35 may beused to determine whether a fault within the power generation system 18is within the rectifier circuit or outside of the rectifier 25 such aswithin the alternator 20 or the power lines 22. Determining whether afault is within the rectifier may reduce the time required to locate andrepair the fault and return the machine 10 to operation.

It will be appreciated that the foregoing description provides examplesof the disclosed system and technique. However, it is contemplated thatother implementations of the disclosure may differ in detail from theforegoing examples. All references to the disclosure or examples thereofare intended to reference the particular example being discussed at thatpoint and are not intended to imply any limitation as to the scope ofthe disclosure more generally. All language of distinction anddisparagement with respect to certain features is intended to indicate alack of preference for those features, but not to exclude such from thescope of the disclosure entirely unless otherwise indicated.

Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context.

Accordingly, this disclosure includes all modifications and equivalentsof the subject matter recited in the claims appended hereto as permittedby applicable law. Moreover, any combination of the above-describedelements in all possible variations thereof is encompassed by thedisclosure unless otherwise indicated herein or otherwise clearlycontradicted by context.

The invention claimed is:
 1. A system for detecting faults in arectifier comprising: an AC current generator for generating an ACoutput current; a rectifier operatively connected to receive the ACoutput current to define an AC input current to the rectifier andtransform the AC input current into a DC output current, the rectifierincluding at least one diode; a first current sensor for generatingfirst current signals indicative of the AC input current; an outputcurrent sensor for generating output current signals indicative of theDC output current; a controller configured to: store a predetermineddifference threshold; receive the first current signals from the firstcurrent sensor; determine the AC input current based upon the firstcurrent signals; generate a simulated rectified DC input current basedupon the AC input current; receive the output current signals from theoutput current sensor; determine the DC output current based upon theoutput current signals; compare the simulated rectified DC input currentto the DC output current; and generate an alert command if a differencebetween the simulated rectified DC input current and the DC outputcurrent exceeds the predetermined difference threshold.
 2. The system ofclaim 1, wherein the AC current generator generates three-phase ACoutput current including a first phase, a second phase, and a thirdphase.
 3. The system of claim 2, further including a second currentsensor and a third current sensor, and wherein the first current sensoris operatively connected to the first phase, the second current sensoris operatively connected to the second phase, and the third currentsensor is operatively connected to the third phase.
 4. The system ofclaim 2, wherein the rectifier includes a three-phase full wave bridgerectifier circuit.
 5. The system of claim 2, wherein the rectifiertransforms the three-phase AC input current into a single DC outputcurrent along a single DC output line.
 6. The system of claim 2, whereinthe controller is further configured to determine a sum of an absolutevalue of the first phase, the second phase, and the third phase of theAC output current.
 7. The system of claim 6, wherein the controller isfurther configured to apply a scaling factor to the sum before comparingthe simulated rectified DC input current to the DC output current. 8.The system of claim 2, wherein the controller is further configured todetermine a positive sequence set of equal currents, a negative sequenceset of equal currents, and a zero sequence set of equal currents basedupon the AC input current, and determine whether a short circuit existsin one of a plurality of diodes of the rectifier based upon the positivesequence set of equal currents, the negative sequence set of equalcurrents, and the zero sequence set of equal currents.
 9. The system ofclaim 1, wherein the AC current generator is an alternator.
 10. Thesystem of claim 1, wherein controller is further configured to store athreshold period of time and generate the alert command if thedifference exceeds the predetermined difference threshold for longerthan the threshold period of time.
 11. The system of claim 1, whereinthe rectifier including a plurality of diodes and the controller isfurther configured to determine a positive sequence set of equalcurrents, a negative sequence set of equal currents, and a zero sequenceset of equal currents based upon the AC input current, and determinewhether a short circuit exists in one of the plurality of diodes basedupon the positive sequence set of equal currents, the negative sequenceset of equal currents, and the zero sequence set of equal currents. 12.A controller-implemented method of detecting faults in a rectifierincluding at least one diode, comprising: storing a predetermineddifference threshold; generating an AC output current; providing the ACoutput current to the rectifier to define an AC input current;transforming the AC input current into a DC output current through therectifier; receiving first current signals from a first current sensorindicative of the AC input current; determining the AC input current tothe rectifier based upon the first current signals; generating asimulated rectified DC input current based upon the AC input current;receiving output current signals from an output current sensorindicative of the DC output current from the rectifier; determining theDC output current based upon the output current signals; comparing thesimulated rectified DC input current to the DC output current; andgenerating an alert command if a difference between the simulatedrectified DC input current and the DC output current exceeds thepredetermined difference threshold.
 13. The method of claim 12, furtherincluding generating three-phase AC input current including a firstphase, a second phase, and a third phase.
 14. The method of claim 13,further including receiving second current signals from a second currentsensor indicative of an AC input current of the second phase,determining the AC input current of the second phase based upon thesecond current signals, receiving third current signals from a thirdcurrent sensor indicative of an AC input current of the third phase, anddetermining the AC input current of the third phase based upon the thirdcurrent signals.
 15. The method of claim 13, further includingdetermining a sum of an absolute value of the first phase, the secondphase, and the third phase of the AC output current.
 16. The method ofclaim 15, further including applying a scaling factor to the sum beforecomparing the simulated rectified DC input current to the DC outputcurrent.
 17. The method of claim 13, further including determining apositive sequence set of equal currents, a negative sequence set ofequal currents, and a zero sequence set of equal currents based upon theAC input current, and determining whether a short circuit exists in oneof a plurality of diodes based upon the positive sequence set of equalcurrents, the negative sequence set of equal currents, and the zerosequence set of equal currents.
 18. The method of claim 12, furtherincluding storing a threshold time and generating the alert command ifthe difference exceeds the predetermined difference threshold for longerthan the threshold time.
 19. The method of claim 12, wherein therectifier including a plurality of diodes and the controller is furtherconfigured to determine a positive sequence set of equal currents, anegative sequence set of equal currents, and a zero sequence set ofequal currents based upon the AC input current, and determine whether ashort circuit exists in one of the plurality of diodes based upon thepositive sequence set of equal currents, the negative sequence set ofequal currents, and the zero sequence set of equal currents.
 20. Amachine comprising: an AC current generator for generating an AC outputcurrent; a prime mover operatively connected to the AC currentgenerator; a rectifier operatively connected to receive the AC outputcurrent to define an AC input current to the rectifier and transform theAC input current into a DC output current, the rectifier including atleast one diode; a first current sensor for generating first currentsignals indicative of the AC input current; an output current sensor forgenerating output current signals indicative of the DC output current; acontroller configured to: store a predetermined difference threshold;receive the first current signals from the first current sensor;determine the AC input current based upon the first current signals;generate a simulated rectified DC input current based upon the AC inputcurrent; receive the output current signals from the output currentsensor; determine the DC output current based upon the output currentsignals; compare the simulated rectified DC input current to the DCoutput current; and generate an alert command if a difference betweenthe simulated rectified DC input current and the DC output currentexceeds the predetermined difference threshold.